1. Field of the Invention
This invention relates to methods of removing biuret from biuret-containing urea and, in particular, to methods for producing urea of reduced biuret content and for recovering biuret from biuret-containing urea.
2. Description of the Art
Urea is a widely used fertilizer and chemical precursor. Most often it contains some biuret that forms during the urea manufacturing process or when urea is otherwise heated to 130.degree. C. or above. Biuret can interfere with chemical processing and is toxic to many plants. Its phytotoxicity has been thoroughly studied, and it is regulated and monitored by government agencies and industry. For instance, the Indian government prohibits the import of urea containing more than 2 weight percent biuret. The United States agricultural industry generally observes an upper limit of 0.25 weight percent biuret for urea fertilizers classified as "low biuret." This criterion is generally recognized by the citrus and other industries that use urea for foliar fertilization.
Detectable biuret toxicity symptoms have been noted in field tests on lemon and grapefruit in Southern California at biuret levels as low as 0.1 weight percent. Biuret toxicity has also been observed with topically applied urea prills and solutions. Seed germination inhibition and damage to seedlings has been observed in wheat, barley and similar grain crops at levels of 2 weight percent biuret.
Damage to corn has been observed at foliar biuret dosages of 0.2 to 0.5 kilogram per hectare. Thirty percent yield loss was noted in one study at 1.7 kilograms biuret per hectare banded near seeds. Wheat damage has been observed at 0.2 to 0.5 kilogram per hectare foliarly applied, and severe toxicity was observed at 6.0 kilograms per hectare biuret banded in the soil. Fifteen to twenty ppm. soil biuret level has been shown to inhibit barley seed germination while substantial crop damage from foliar application often occurs at 0.4 to 0.6 kilogram biuret per hectare.
Similar effects have been observed in rice, citrus, cotton, avocado, beans, soybeans and potatoes, several of which are particularly sensitive to biuret in foliar fertilizers. In citrus, as little as 0.2 kilograms foliarly applied biuret per hectare causes detectable damage. Avocados are damaged by as little as 50 ppm. biuret in foliar sprays. As little as 3 kilograms per hectare biuret banded in the soil inhibits potato germination and causes citrus damage in light soils. These studies, and a comprehensive review of the literature available on this subject, are presented by Mithyantha, Kulkarni, Tripathi and Agnihothrudu, Fertilizer News, 1977, pp. 13-18.
In view of these results, it is not surprising that the industry has devoted substantial effort to methods of preventing biuret formation in the first instance, and to methods of reducing its concentration once it is formed. Most contemporary commercial urea plants are capable of producing solid and solution urea containing much less biuret than was previously the case. However, essentially all commercial ureas contain at least 0.5 weight percent biuret, and most contain from 1 to 2 weight percent biuret. Biuret content can rise considerably higher if manufacturing conditions are not adequately controlled.
On the other hand, biuret is not without its value. It is widely used in commerce as a precursor for pharmaceuticals, herbicides, and other compounds, as an analytical reagent, and as a ruminant feed supplement. All of these utilities benefit from (if not require) the use of relatively pure biuret.
While biuret can be produced by several chemical methods, it is typically obtained by pyrolyzing urea at a temperature of at least 130.degree. C. and for a period of time sufficient to convert at least a portion of the urea to biuret. An illustrative urea pyrolysis process is discussed by Shipley and Watchorn in British patent No. 1,156,099. As disclosed by Shipley et al., the method produces a mixture of urea, biuret and higher molecular weight urea condensation products such as triuret, cyanuric acid, ammelide, melamine, ammonium cyanurate, methylene diurea, and/or other compounds.
Urea manufactured as solid prills is often treated at temperatures that result in some conversion of urea to biuret and, in many cases, the formation of higher molecular weight compounds as well. While the biuret concentration in prilled ureas is typically low, e.g., 0.5 to 3 weight percent, the amount of biuret contained in such products is substantial due to the large volume of prilled urea manufactured annually. Many of the commercial biuret-containing prilled ureas also contain higher molecular weight urea condensation products such as those mentioned above.
Many of the higher molecular weight condensation products present in some ureas appear to form by the reaction of urea with itself or with previously formed condensation products, or by reactions of, or between, previously formed condensation products. Others, such as methylene diurea, appear to form by the reaction of urea and/or condensation products with additives or other impurities such as formaldehyde which is sometimes employed as a urea anti-caking agent. Regardless of their origin, one or more of such impurities are known to exist in biuret obtained from urea by presently available methods as discussed by Shipley et al., supra, and Kaasenbrood in U.S. Pat. No. 3,185,731.
While urea pyrolysis and prilled urea manufacture afford an ample supply of biuret, the major utilities for biuret benefit from the use of that compound in relatively pure form. Analytical procedures and pharmaceutical and herbicide manufacturing practices involving the use of biuret are most often unacceptably complicated by the presence of higher molecular weight condensation products, and the biuret dosage rate which can be employed in ruminant feed supplements is often limited by the toxicity of such impurities.
Methods presently available for commercially recovering biuret from urea typically involve low temperature crystallization procedures such as those discussed by Shipley et al. and Kaasenbrood, supra, in which the urea and/or biuret are recrystallized several times and separated, and the solid phase is washed to obtain purified urea and/or biuret. While such methods effectively separate biuret from urea, such separation is not complete, and the methods involve expensive, low temperature recrystallization procedures. A substantial amount of biuret generally remains in the urea fraction, and the biuret fraction typically contains minor amounts of urea, even after repeated recrystallization. Furthermore, such procedures do not efficiently separate biuret from urea and cogeneric impurities such as higher molecular weight urea condensation products. Typically, some or all of the higher molecular weight impurities remain in the biuret fraction.
Several authors have disclosed that biuret can be removed from urea by contacting an aqueous biuret-containing solution with the hydroxide ion form of an anion exchanger. For instance, Fuentes et al., U.S. Pat. No. 3,903,158, disclose that impurity biuret can be removed from aqueous urea solutions by ion exchange. Takahashi et al., "Determination of Biuret in Urea by Ion Exchange Resins," Soil and Plant Food, Vol. 3, No. 3, January 1958, pgs. 142-144, disclose that biuret can be separated from aqueous urea solutions and that the biuret retained on the ion exchanger can be eluted to obtain an indication of the biuret concentration in the urea feed.
While such processes are useful for purifying urea, their use for the recovery of biuret from aqueous urea solutions suffers from several disadvantages. Biuret is hydrolyzed and destroyed by the highly basic anion exchangers employed by Fuentes et al. and Takahashi et al. Furthermore, the maximum biuret concentration which could be achieved in an ion exchanger regenerant, such as that employed by Fuentes et al. or Takahashi et al., even in the absence of significant biuret hydrolysis, is at most 0.5 weight percent, and that could occur only in the very initial stages of regeneration. The average biuret concentration in the total regenerant is typically well below 0.1 weight percent. This is because the volume of regenerant typically employed to restore the initial biuret-retaining ability of an ion exchanger, i.e., for regeneration, is so large that the biuret concentration in the total regenerant effluent is much lower than 0.1 weight percent. Almost without exception, it is preferable to completely regenerate ion exchangers to assure the greatest capability for removing the exchanged substance (in this case biuret) in subsequent cycles. Complete regeneration is more readily accomplished by the use of large regenerant volumes. In addition, the use of alkaline regenerants as disclosed by Fuentes et al. destroys biuret as disclosed in our U.S. Pat. No. 4,345,099 for Method of Selectively Removing Biuret from Urea and Compositions for Use Therein, the disclosure of which is incorporated herein by reference in its entirety.
The highest biuret concentrations, if any, obtainable in the exchanger regenerants of Fuentes et al. and Takahashi et al. render those solutions impractical for a variety of reasons. (The strongly alkaline regenerants of Fuentes et al. may not contain any biuret at all unless they are neutralized and/or cooled to prevent biuret hydrolysis.) The solubility of biuret in water at 0.degree. C. is 0.53 weight percent. (Urea, Its Properties and Manufacture, Chao, Chao's Institute, West Covina, Calif., Library of Congress Catalogue Card No. Ai-11524.) Thus, biuret could not be crystallized from solutions of such low biuret concentration unless the solutions were modified to substantially depress their freezing points, and such modification of the solutions might itself prevent biuret crystallization even at lower temperatures. Thus, the commercial use of such solutions would require the shipment of large volumes of water with the attendant cost of such shipment, or evaporation of enough of the water to significantly increase biuret concentration. Obviously, such evaporation adds further expense to the process.
A further disadvantage associated with the prior art methods for separating biuret from urea involves the presence of a significant proportion of higher molecular weight urea condensation products in a substantial portion of biuret-containing ureas. While the prior art crystallization processes could be employed to separate biuret from some of the higher molecular weight urea condensation products, those processes, as mentioned above, require time-consuming, repeated low temperature recrystallization. The expense involved in such methods obviously increases the cost of biuret derived from such sources and limits its application as a result. For instance, ruminant feed supplement manufacturers generally choose to use relatively impure, less expensive biuret at dosage rates which are sufficiently low to avoid the toxic effects of the higher molecular weight impurities. Neither Fuentes et al. nor Takahashi et al. mention the presence of materials other than urea and biuret or the possibility that impurity-free biuret can be recovered from urea solutions which contain higher molecular weight urea condensation products. In fact, Takahashi et al. observe that "usually, urea for agriculture does not contain nitrogen compounds other than biuret." While that is often the case, some urea solutions, and in particular those formed from urea which has been pyrolyzed at temperatures above 130.degree. C. for any significant period of time, contain a significant proportion of urea condensation products of higher molecular weight than biuret, some of which are toxic, and all of which can impair product utility.
The use of strongly basic anion exchangers to remove biuret from urea as disclosed by Fuentes et al. and Takahashi et al., supra, suffers from several further disadvantages. Strongly basic anion exchangers such as Amberlite IRA-400 cost in the range of about $50 to about $150 per cubic foot. The strongly caustic or acidic solutions used to regenerate the exchangers are also relatively expensive. Since, according to the literature, the biuret is relatively strongly held by the anion exchanger (a feature which would be beneficial from the standpoint of assuring adequate removal of biuret from the urea solution), the art suggests that relatively severe regeneration conditions are required to efficiently remove the biuret from the deactivated anion exchanger. Obviously, the cost of anion exchanger regeneration, the cost of constructing, maintaining and operating a system capable of removing biuret from a certain quantity of urea solution, and the expense of the anion exchanger required in the process, all increase as the frequency and/or severity of regeneration increases. Thus, the requirement for frequent and/or more severe regeneration increases regenerant costs and the amount of anion exchanger and the size of the operating facility required to treat a given amount of urea solution.